Re: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop detach and loop open

Yu Kuai
2024-05-22T02:31:13Z

Hi,

在 2024/05/22 9:39, Jens Axboe 写道:
> On 5/21/24 4:42 PM, Gulam Mohamed wrote:
>> Description
>> ===========
>>
>> 1. Userspace sends the command "losetup -d" which uses the open() call
>> to open the device
>> 2. Kernel receives the ioctl command "LOOP_CLR_FD" which calls the
>> function loop_clr_fd()
>> 3. If LOOP_CLR_FD is the first command received at the time, then the
>> AUTOCLEAR flag is not set and deletion of the
>> loop device proceeds ahead and scans the partitions (drop/add
>> partitions)
>>
>> if (disk_openers(lo->lo_disk) > 1) {
>> lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
>> loop_global_unlock(lo, true);
>> return 0;
>> }
>>
>> 4. Before scanning partitions, it will check to see if any partition of
>> the loop device is currently opened
>> 5. If any partition is opened, then it will return EBUSY:
>>
>> if (disk->open_partitions)
>> return -EBUSY;
>> 6. So, after receiving the "LOOP_CLR_FD" command and just before the above
>> check for open_partitions, if any other command
>> (like blkid) opens any partition of the loop device, then the partition
>> scan will not proceed and EBUSY is returned as shown in above code
>> 7. But in "__loop_clr_fd()", this EBUSY error is not propagated
>> 8. We have noticed that this is causing the partitions of the loop to
>> remain stale even after the loop device is detached resulting in the
>> IO errors on the partitions
>>
>> Fix
>> ---
>> Re-introduce the lo_open() call to restrict any process to open the loop
>> device when its being detached
>>
>> Test case
>> =========
>> Test case involves the following two scripts:
>>
>> script1.sh
>> ----------
>> while [ 1 ];
>> do
>> losetup -P -f /home/opt/looptest/test10.img
>> blkid /dev/loop0p1
>> done
>>
>> script2.sh
>> ----------
>> while [ 1 ];
>> do
>> losetup -d /dev/loop0
>> done
>>
>> Without fix, the following IO errors have been observed:
>>
>> kernel: __loop_clr_fd: partition scan of loop0 failed (rc=-16)
>> kernel: I/O error, dev loop0, sector 20971392 op 0x0:(READ) flags 0x80700
>> phys_seg 1 prio class 0
>> kernel: I/O error, dev loop0, sector 108868 op 0x0:(READ) flags 0x0
>> phys_seg 1 prio class 0
>> kernel: Buffer I/O error on dev loop0p1, logical block 27201, async page
>> read
>>
>> V1->V2:
>> Added a test case, 010, in blktests in tests/loop/
>> Signed-off-by: Gulam Mohamed <gulam.mohamed@oracle.com>
>> ---
>> drivers/block/loop.c | 19 +++++++++++++++++++
>> 1 file changed, 19 insertions(+)
>>
>> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
>> index 28a95fd366fe..9a235d8c062d 100644
>> --- a/drivers/block/loop.c
>> +++ b/drivers/block/loop.c
>> @@ -1717,6 +1717,24 @@ static int lo_compat_ioctl(struct block_device *bdev, blk_mode_t mode,
>> }
>> #endif
>>
>> +static int lo_open(struct gendisk *disk, blk_mode_t mode)
>> +{
>> + struct loop_device *lo = disk->private_data;
>> + int err;
>> +
>> + if (!lo)
>> + return -ENXIO;
>> +
>> + err = mutex_lock_killable(&lo->lo_mutex);
>> + if (err)
>> + return err;
>> +
>> + if (lo->lo_state == Lo_rundown)
>> + err = -ENXIO;
>> + mutex_unlock(&lo->lo_mutex);

This doesn't fix the problem completely, there is still a race window.

lo_release
if (disk_openers(disk) > 0)
reutrn
-> no openers now
lo_open
if (lo->lo_state == Lo_rundown)
-> no set yet
open succeed
mutex_lock(lo_mutex)
lo->lo_state = Lo_rundown
mutex_unlock(lo_mutex)
__loop_clr_fd

And with the respect, loop_clr_fd() has the same problem.

I think probably loop need a open counter for itself.

Thanks,
Kuai

>> + return err;
>> +}
>
> Most of this function uses spaces rather than tabs.
>